1. Field of the Invention
The present invention relates to an electroluminescence element.
2. Description of the Related Art
Since an electroluminescence (hereafter, it may be abbreviated as “EL”) element having a light emitting layer interposed between a pair of electrodes for emitting a light beam by applying a voltage between the electrodes has a high brilliance among various display elements and a high response speed, it attracts the attention so as to promote its use in practice.
For the production of the EL element, a method of successively laminating an anode layer, an EL layer and a cathode layer onto a substrate is adopted. In actually forming the EL layer, various layers comprising the EL layer are successively laminated.
Among the above-mentioned layers, it is efficient to laminate by sputtering the electroconductive layers such as the anode layer and the cathode layer in terms of production. However, at the time of laminating the cathode layer, since the base is the EL layer, the EL layer performance may easily be deteriorated because the EL layer is exposed to sputtering particles having high energy or to the plasma atmosphere in the case plasma is used.
The material comprising the EL layer of the EL element is vulnerable to the physical or chemical environmental change and often generates the dark spots. To prevent permeation of the moisture content in the air, one of the cause to generate dark spots, the entire EL element may be covered with a protection film for shielding the moisture content (Japanese Patent Application Laid-Open (JP-A) No. 2003-338368).
A method of avoiding introduction of the moisture content or the oxygen in the air to the cathode comprising an ITO, or the like by providing the cathode of the organic EL element comprising an electron injecting electrode layer (made of such as Mg, Ca, or Ba) and an amorphous transparent electroconductive layer (made of a In—Zn—O based oxide film) from the organic light emitting layer side has been proposed (JP-A No. 10-162959).
A method of successively laminating a first electrode, a sputtering protection layer and a second electrode on a substrate, in which the sputtering protection layer is made of gold, nickel or aluminum has also been proposed (JP-A No. 2003-77651).
A method of providing a cathode layer of the organic EL element in a two layer structure, in which an electron transporting protection layer is provided between the two layers as a layer of a mixture of an electron transporting organic material such as a BCP (bathocuproin) and Li, Cs, Ba, Sr, Ca, or the like as an alkaline metal or an alkaline earth metal has also been proposed (JP-A No. 2004-127740).
A cathode structure of an organic EL element containing a thin metal layer covered with a wide band gap semiconductor (such as ZnSe) to be directly contacted with an organic EL region has also been proposed (JP-A No. 10-223377).
Alternatively, the organic EL layer of an organic EL light emitting element that comprises a substrate, a cathode, an organic EL layer, and an upper transparent anode has been proposed. Here, the organic EL layer comprises an organic light emitting layer and a hole injecting layer, in which the hole injecting layer contacts with the upper transparent anode and has a 30 to 1,000 nm thickness (JP-A No. 2004-227943).
Furthermore, a buffer structure comprising a first buffer layer containing an alkaline halide and a second buffer layer containing a phthalocyanine formed between a light emitting layer and a sputtering cathode layer has been proposed (JP-A No. 2002-75658).
A buffer layer containing a phthalocyanine compound with at least one metal selected from the group consisting of Au, Pt, Pd and Ag, and formed between an organic El layer and an upper electrode has also been proposed (JP-A No. 2004-296234).
Although JP-A No. 2003-338368 discloses that the cathode already laminated at the time of forming the protection film of a high melting point metal on the cathode (second electrode) provides a buffer so that the protection film can be formed by sputtering, any measure for solving the deterioration of the EL layer at the time of laminating the cathode is not shown.
In JP-A No. 10-162959, although the electron injecting electrode layer as the lower layer may produce the protection effect at the time of forming an amorphous transparent electroconductive film on the electron injecting electrode layer by sputtering, a problem of transmittance decline caused when a sufficient thickness to perform the protection effect is provided to the layer cannot be avoided.
JP-A No. 2003-77651 discloses an issue of solving the deterioration of the EL layer at the time of laminating the second electrode. However, it has been difficult to achieve both the necessary protection function and transparency because, in the case of using a metal, a thicker layer which is preferable in terms of the protection function adversely deteriorates the transparency.
In JP-A No. 2004-127740, the cathode layer has a three layer structure including the electron transporting protection layer, and a metal is mixed. Accordingly, a problem is involved in that the protection effect and the transmittance can hardly be achieved at the same time. Moreover, due to the introduction of the alkaline metal or the alkaline earth metal, stability of the electron transporting protection layer may be deteriorated.
As to JP-A No. 10-223377, transparency cannot be obtained sufficiently with a cathode structure comprising a wide band gap semiconductor such as ZnSe and a thin metal layer.
As to JP-A No. 2004-227943, although the hole injecting layer may perform the protection function at the time of forming the upper transparent electrode, the problem at the time of forming the cathode after the organic light emitting layer is not touched upon.
Furthermore, JP-A No. 2002-75658 discloses that the damage applied to the organic layer such as the light emitting layer at the time of sputtering the cathode can be reduced. However, a problem is involved in that the driving voltage is made higher if the film thickness of the second buffer layer containing a phthalocyanine is made thicker for improving the protection effect. Moreover, since the phthalocyanine absorbs a visible light beam (blue to green), a problem of the transmittance decline is also involved.
JP-A No. 2004-296234 discloses that the damage to the organic EL layer at the time of forming the upper electrode can be alleviated. Nonetheless, since the buffer layer contains a phthalocyanine compound and a metal is doped to the phthalocyanine compound, as in the above-mentioned case, a problem of the transmittance decline is also involved.